With the continuous development of nanoscience and technology, nanomaterials are increasingly being applied into lubricating systems, thus emerging the research area of nano-lubricating additives. The most commonly used nano-lubricating additives may be divided into five types: (1) zero-dimensional (in a spherical or spherical-like shape) solid nanoparticles, such as silicon dioxide nanoparticles, metal nanoparticles, and metal oxide nanoparticles; (2) zero-dimensional (in a spherical or spherical-like shape) hollow nanoparticles, such as fullerene and its derivatives, and WS2/MoS2 hollow nanospheres; (3) one-dimensional nanoparticles, such as carbon nanotubes, ZnS/WS2/MoS2 metal sulfide nanorods, and Mo—S—I/Mg2B2O5 nanowires; (4) two-dimensional layered nanoparticles, such as disulfide metal salts, clays, layered double hydroxides (LDHs), layered metal phosphates, graphite and its derivatives, and graphene and its derivatives; (5) other types of nanomaterials, such as inorganic-organic hybridized surfactants, represented by amphipathic polyoxometalate-organic hybrids.
Nanoparticles have advantages such as small particle size, large specific surface area and high surface energy, resulting in tendencies of agglomeration, aggregation and sedimentation in a solution. It has become one of main technical difficulties limiting the nanomaterials to be applied as the lubricating additives due to their poor dispersion stabilities in lubricating oils. In general, it is necessary to disperse inorganic nanoparticles in oils by means of modifying particles surfaces or adding dispersing agents, since the inorganic nanoparticles are hard to be dissolved in oil. For example, in a practical application, metal nanoparticles have oxygen-containing groups on their surface for further surface modification, such as hydroxyl and carboxyl, resulting from the inevitable exposure of air at their surfaces. In this case, in order to well disperse the metal nanoparticles in the oil medium, it is the most direct method to add oil-soluble surfactants, such as long chain hydrocarbyl carboxylic acids and long chain hydrocarbyl sulphonates. The surfactants allow for the dispersion of the metal nanoparticles in the oil medium by means of intermolecular dipole interactions, van der Waals force or electrostatic interactions with oxidized surfaces of the metal nanoparticles. However, these temporally dispersed metal nanoparticles by adding the surfactants are incapable of being stable in the oil medium for a long period, as the above-mentioned intermolecular dipole interactions, van der Waals forces and electrostatic interactions are weak, therefore aggregation and sedimentation eventually occur. Another commonly used method for dispersing metal nanoparticles is to add oil-soluble coordination compounds during synthesizing the metal nanoparticles. In specific, metal ion precursors are initially subjected to the coordination reaction with corresponding oil-soluble coordination compounds, and then the metal nanoparticles are synthesized by adding reducing agents. The metal nanoparticles obtained by this method are stable in the oil medium. However, this method is costly, which limits the application of the such-synthesized metal nanoparticles.
Another example is layered metal phosphate nanosheets, e.g., α-zirconium phosphate (α-ZrP, Zr(HPO4)2.H2O) nanosheets, in which a layered structure is formed with the connection between oxygen atoms in the phosphate groups and the zirconium atoms. In specific, three of the oxygen atoms for each phosphate group are involved in formation of the layered structure, and the rest oxygen atom in the forms of a hydroxyl group extends towards the interlayer of the layered structure. As the hydroxyl group carried by α-ZrP tends to be partially deprotonated in an aqueous solution, α-ZrP is partially soluble in water, thus resulting in weak acidity in the aqueous solution. However, α-ZrP is insoluble and hard to disperse in the oil media, which leads to a key technical issue to be resolved when applying α-ZrP nanosheets in the lubricating oils. Nevertheless, there are few literatures so far reporting the possibility in properly resolving this issue. It is reported that nonionic surfactants containing oxyethylene groups may be capable of dispersing layered zirconium phosphate nanosheets treated by HP in both an aqueous solution and organic solutions. However, this method has disadvantages as described below. On one hand, HP used is very dangerous, and on the other hand, zirconium phosphate treated by HP will inevitably adsorb HF, which, as well known, badly erodes metallic materials due to hygroscopicity of α-ZrP. Accordingly, it is impossible to apply zirconium phosphate treated by HF to the lubricating oil. It has also been reported that it is safer to use polyamine-modified polyisobutylene succinimide (PIBSA-PAM) as the dispersing agent for dispersing α-ZrP nanosheets in the lubricating oil. However, it is impossible for this method to form a long-term stable emulsion after mixing PIBSA-PAM, α-ZrP and the lubricating oil, resulting in liquid phase separation inevitably. Accordingly, this static instability decreases effectiveness of the α-ZrP nanosheets and causes some problems when applying in practice.
Therefore, it still needs to further improve dispersion stability of nanomaterials in the oil medium.